The MAXI/GSC catalog in the low Galactic-latitude sky
Takafumi Hori,1 Yoshihiro Ueda,1 Megumi Shidatsu,2Taiki Kawamuro,1 Atsushi Tanimoto,1 Saeko Oda,1 and the MAXI team
1Department of Astronomy, Kyoto University, Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
2
MAXI team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
E-mail(TH): hori@kusastro.kyoto-u.ac.jp
Abstract

Complete X-ray source catalogs in our Galaxy are the basis to study the demography of Galactic X-ray
populations. Since MAXI/Gas Slit Camera (GSC) covers the 2–10 keV band with the highest sensitivity
as an all-sky mission, its Galactic source catalog will be complementary to those of hard X-ray (>10 keV)
missions. Here we present the latest X-ray source catalog of MAXI/GSC in the low Galactic-latitude sky(|b| < 10◦ ). We utilize the same image fitting method as that in Hiroi et al. (2013), who constructed
the 37-month MAXI/GSC catalog at high Galactic latitudes (|b| > 10◦ ). To overcome source confusion incrowded regions, we have accurately calibrated the position-dependent shape of the point spread functionof MAXI/ GSC by utilizing the onboard data. We also take into account the Galactic ridge X-ray emission
in the background, utilizing the model by Revnivtsev et al. (2006). With a preliminary analysis, we detect
149 sources with significance above 8σ, among which about 71% have counterparts in the Swift/BAT
70-month catalog.
Key words: workshop: surveys — X-rays: catalog

1. Introduction
MAXI/Gas Slit Camera (GSC) has achieved the best
sensitivity in the energy band of 2 − 10 keV among all
past or on-going all-sky X-ray missions. All-sky X-ray
source catalogs of MAXI/GSC are complementary toother ones covering different energy bands, such as thoseof ROSAT (< 2 keV), Swift (> 14 keV), and INTEGRAL (> 15 keV). The 37 month MAXI/GSC catalog
in the high Galactic-latitude sky (|b| > 10◦ ) is publishedby Hiroi et al. (2013). At low Galactic latitudes, source
confusion becomes more significant because the Galactic plane is crowded by bright X-ray sources. Also, weneed to take into account the the Galactic ridge X-ray
emissions (GRXEs) in modeling the background. In thispaper, we report the current results of the MAXI/GSC
catalog in the low Galactic-latitude sky.
2. Data Reduction and Analysis
We utilize the first 37-month MAXI/GSC data observed
from 2009 September 23 to 2012 October 15. The Galactic center region (l < 30◦ , 330◦ < l, and |b| < 5◦ ) isexcluded, where source confusion is too serious. Data
reduction is performed with the same procedures described in Hiroi et al. (2013). We use our latest model ofthe non X-ray background (NXB) plus the cosmic X-raybackground (CXB), which is detailed in Shidatsu et al.
(2017, this conference).
In addition to the NXB and CXB, we consider theGRXE, whose surface density rapidly increases toward
the Galactic plane (b = 0). We model the GRXE profile with the ellipsoid model by Revnivtsev et al. (2006),
which consists of the Galactic disk and bulge/bar components.
3.

Current Results

To determine the significance and flux of each X-ray
source, we perform Poisson maximum-likelihood fit tothe observed image with a model consisting of source
point spread functions (PSFs) and the background(NXB+CXB+GRXE). We first only consider the sourcesof the Swift/BAT 70-month catalog by fixing their positions. Then, from the residual image, we search forsources detected only with MAXI/GSC.
As a result, we detect 149 sources with significanceabove 8 σ at |b| < 10◦ , among which 107 (71%) are in theSwift/BAT 70-month catalog. The number of sourcesin each category is summarized in Table 1. Figure 1
shows the locations of the detected sources in Galactic
coordinates, including the high Galactic-latitude sourcesin Hiroi et al. (2013). Figure 2 shows the log N -log

Number of Sources
11119611645(25)
(6)
4442

S relation in the low Galactic-latitude (|b| < 10◦ ) sky
outside the Galactic center region, where the numberdensity N of all sources with 4–10 keV fluxes above S isplotted. The slope is close to unity.
4.

10−3

CategoryGalaxies
Galaxy clusters
Seyfert galaxies
Quasars
Cataclysmic variables
Stars
X-ray binaries(Neutron star)(Black Hole)
Pulsars
Supernova remnants
Unmatched

N (>S) [deg−2]

Table 1. Categories of Cataloged Sources

0.010.1

Fig. 1. The locations of all MAXI sources in Galactic coordinates, including those of Hiroi et al. (2013) obtained from the 37 month data
at high Galactic latitudes. Each radius is proportional to the logarithm of the flux. The Different colors correspond to different types;
galaxies (cyan), galaxy clusters (green), Seyfert galaxies (blue), quasars (purple), cataclysmic variables (purple), stars (orange), X-ray
binaries (red), pulsars (black), supernova remnants (pink) and X-ray sources without counterpart in Swift/BAT 70-month catalog (Gray).

10−1210−1110−1010−910−8−2−1S (4−10 keV) [ergs cm s ]Fig. 2. The log N –log S relationof the MAXI/GSC sources in theGalactic plane region.

PSFs as accurately as possible. We are re-calibrating
the PSF profiles by referring to the onboard data indetail. This new PSF database will be utilized for all
future works of catalog production (including that at
high Galactic-latitudes) and light curve extraction fromMAXI/GSC data.

Future WorkWe are working to produce a MAXI/GSC low Galacticlatitude catalog utilizing the 7-year data (Hori et al.
2017, in preparation), which will update the results presented here. To overcome the difficulties in detecting
faint objects around bright sources, we must model the10−7References
Revnivtsev et al. 2006, A&A, 452, 169
Hiroi et al. 2013, ApJS, 207, 36